Government vets are trying to establish whether hundreds of turkeys on a poultry farm had been killed by bird flu.
Experts were called to the farm near Lowestoft, Suffolk, late on Thursday following the outbreak of a mystery illness, said the Department of Environment, Food and Rural Affairs (Defra).
A spokeswoman said officials hoped to have the preliminary results of tests by Friday night.
She said vets were stopping traffic going in or out of the farm - but no restriction zone had been set up. "We are currently investigating a report of unexplained illness on a poultry farm," she said.
"Reports of the illness were received late last night (Thursday) and the premise was placed under restrictions immediately.
"A full investigation began ... with samples being sent ... for testing. Initial results will be available shortly."
Around 1,000 turkeys are thought to have died.
The spokeswoman said other birds remained on the farm.
In March 2006, a wild swan found in Cellardyke, Fife, was found to have the H5N1 version of the virus, which had been responsible for the deaths of more than 100 people, mostly in Asia.
In April 2006, more than 30,000 birds were slaughtered after chickens on a poultry farm near Dereham, Norfolk, tested positive for the H7 strain of bird flu - which poses less risk to humans.
